1. ax-2ay+2bx-4by=ax+2bx-（2ay+4by） =（x-2y)(a+2b)
2. x^3+x^2-4x-4=x(x^2-4)+(x^2-4) =(x-1)(x-2)(x+2)
3. x^2-x-4y^2+2y=(x^2-4y^2)-(x-2y)=(x-2y)(x+2y-1)
4. 4xy+1-4x^2-y^2=1-(4x^2-4xy+y^2)=(1+2x-y)(1-2x+y)
5. x^2-10xy+21y^2=(x-3y)(x-7y) 十字交叉法
6. 4a^2b-3ab^2-a^3=a(-a^2+4ab-3b^2)=a(b-a)(a-3b) 十字交叉法
7.x^4-a^2x^2-b^2x^2+a^2b^2==(x^2-b^2)(x^2-a^2)
8. x^2(x+1)-y(xy+x)=x^2(x+1)-yx(y+1)=x(x^2+x)-x(y^2+y)=x[(x^2-y^2)+(x-y)]=x(x+y+1)(x-y)